1. Field of the Invention
The present invention pertains to a ruby and a needle guiding group for a needle printing head, and more particularly to the guiding end structure of such a group.
2. Description of the Prior Art
The needle printing heads are generally comprised of a printing needle guiding group, an electromagnetic group for the actuation of the several needles and an armature retaining group of the printing electromagnets. Typical structures of printing heads are disclosed by several patents; e.g., U.S. Pat. Nos. 4,140,406 and 4,260,270. The printing needle guiding group is generally comprised of an elongated, conical or pyramid shaped nose internally provided with suitable guides able to gradually bend the needles in a direction which is transverse to the longitudinal axis of the cone or pyramid. This bending also shifts the initial circular distribution to an end distribution in parallel columns. In order to generate characters along a printing line sequentially, the head is mounted on a carriage which slides parallel to a platen which supports the printing surface. The characters are then generated by actuating the needles in a selective manner. The needle guides are comprised of pierced diaphragms spaced along the guiding nose axis and are arranged perpendicularly to said longitudinal axis. This is shown in the above-mentioned U.S. patents.
In another embodiment the needle guides are comprised of one or more continuous guiding elements of the type disclosed in U.S. Pat. No. 4,081,067 and by British Pat. No. 1,450,346 which corresponds to U.S. Pat. No. 3,897,865. The guiding ruby of the printing needle writing ends is comprised of two or three alumina plates depending on whether such ends are arranged on one or two parallel columns respectively. Alumina plates, generally rectangularly shaped and having a thickness of about 1 mm are typically joined and fastened by means of epoxy glue in order to form a single rectangular shaped plate. The openings for guiding the needle ends are positioned along the contact line of two contiguous plates. The openings for the needle ends are formed during the preparation of the guiding ruby, by suitably shaping the side of the plate which will be contiguous to the side of a contiguous plate. A grinding operation of the shaped sides follows and then the positioning and the glueing of the plates on these sides is done. Generally, the grinding of the remaining sides of the plates comprising the guiding ruby is not performed so as to avoid a large increase in the cost of the ruby since the cost of the ruby is determined primarily by the machining operations and not the cost of the raw material. Once ready, the ruby is inserted into a housing on the top of the guiding nose and attached therein by means of epoxy glue. This last operation takes a long time and is particularly difficult. It significantly affects the assembling time and therefore the printing head cost. In fact the axis along which the line or the lines of ruby guiding openings are positioned must be perpendicular to the printing plane of the head on the carriage, and consequently to the printing line. This is necessary to avoid both degradation of the printing quality caused by a slope of the printing characters and the stressing of the printing needles which causes their printing ends to bend. The ruby is inserted into the guiding nose housing and suitably fixed to a reference plane, and locked in the correct relative position by means of a pin tool until the epoxy glue is completely cured. This long and expensive operation is required because the nose guiding of the housing must allow the housing of rubies each of which has different characteristics with large tolerances. Therefore, a considerable clearance generally exists between housing and ruby, which prevents the use of the housing as a means of positioning the ruby correctly.